Redox amplification processes have been described, for example in British Specifications Nos. 1,268,126; 1,399,481; 1,403,418; and 1,560,572. In such processes color materials are developed to produce a silver image (which may contain only small amounts of silver) and treated with a redox amplifying solution (or a combined developer/amplifier) to form a dye image.
The developer/amplifier solution contains a color developing agent and an oxidizing agent that will oxidize the color developing agent in the presence of the silver image which acts as a catalyst.
Oxidized color developer reacts with a color coupler to form the dye image. The amount of dye formed depends on the time of treatment or the availability of the color coupler and is less dependent on the amount of silver in the image than is the case in conventional color development processes.
Examples of suitable oxidizing agents include peroxy compounds including hydrogen peroxide and compounds which provide hydrogen peroxide, e.g., addition compounds of hydrogen peroxide such as perborates and addition compounds of hydrogen peroxide with urea. Other oxidizing agents include cobalt (III) complexes including cobalt hexammine complexes; and periodates. Mixtures of such compounds can also be used.
Developer/amplifier solutions are known to deteriorate because they contain both an oxidizing agent (e.g., the peroxide) and a reducing agent (the color developing agent) which react together spontaneously thus leading to loss of activity in a matter of an hour or two.
By contrast conventional photographic color developer solutions which lose activity by aerial oxidation of the color developing agent, typically will be stable for a week or two. In order to reduce oxidation losses, hydroxylamine and its substituted derivatives such as diethylhydroxylamine have been proposed as anti-oxidants for color developer solutions.
Redox developer amplifiers containing hydroxylamine as the antioxidant have been previously shown to be more stable than those containing diethyl hydroxylamine or similar substituted hydroxylamines. However, it has recently been discovered that hydroxylamine in made up developer-replenisher is prone to aerial oxidation. This can lead to variable levels of hydroxylamine in the redox developer and variable sensitometry in the paper print and can be a significant problem in processes that are run under high oxidation conditions such as those in processing machines with partially submerged rollers and with continuous recirculation of developer solution.